A vacuum-arc plasma source has been described in U.S. Pat. No. 4,448,659 in 1984. It contains a cathode in the form of a plate with a large surface area. For the stabilization of the arc the working surface of the plate supports a protective ring which surrounds the cathode. The ring is made of a material on which the formation of cathodic spots is less likely than on the cathode. This plasma source can be used for the production of coatings on large and long articles but it has the following disadvantages:
1. Despite the low probability of the presence of cathodic spots on the protective ring, the cathode material covers the ring as the evaporator works, so that cathodic spots are produced with increasing frequency. This results in the contamination of the coating by the ring material and in ring failure. PA1 2. It is not possible in this apparatus to use a plasma magnetic focusing feature, as this would make the distribution of the cathodic spots on the working surface of the cathode irregular. PA1 3. The cathode in this type of plasma source becomes concave due to its decomposition, and therefore its useful life is short. Moreover, since the working surface of the cathode becomes concave over time, it is practically impossible to use a high-voltage pulse spark igniter in this design of plasma source, so that a mechanical igniter must be used which lowers working reliability and stability.
The absence of a magnetic focusing feature reduces the efficiency of the coating process, and impairs the quality of the coatings because the content of the neutral component (macroparticles, clusters and neutral atoms) in the coating increases.
Information is provided for an electronic arc metal evaporator, as described in Certificate of Authorship No. 307660, M. K.sub.n. C 13/12 of Sep. 9, 1968. This evaporator consists of a housing, a cathode, an igniting electrode and an anode. In order to maintain the stability of the arc the evaporator is provided with a magnetic coil located on the exterior of the housing, made of a nonmagnetic material. The cathode is arranged coaxially inside the housing. The disadvantage of this plasma source is that it can not be used for the application of coatings to large and long articles. The size of the articles that can be coated is limited to 150 mm.
An apparatus of a similar nature is the vacuum-arc plasma source described in "Electric Arc Evaporator of Metals with Magnetic Confinement of Cathodic Spot", L. P. Sablev, et al, Apparatus and Technique for Experiment, 1976, No. 4, pp. 247-249 (in Russian). This vacuum-arc plasma source consists of a cathode with a working surface, a screen, a high-voltage igniting electrode, an auxiliary anode and a primary anode. The cathode is surrounded by a static magnetic stabilizing system which is produced by a magnetic coil. The magnetic force lines of the system are inclined toward the working face. The cathode, the primary and auxiliary anodes, and the screen are arranged coaxially. The apparatus also incorporates a dielectric sleeve. There is a circular gap between the side surface of the cathode and the auxiliary anode. The vacuum-arc plasma source is secured in a water-cooled flange. As this plasma source operates, the magnetic field holds the cathodic spot on the face of the tapered cathode.
This apparatus, like the one described above (see the Certificate of Authorship No. 307660) cannot be used for the application of coatings to large and long articles. For example, a good coating the thickness precision of which is 5 percent can be obtained on an article the size of which does not exceed 150 mm at a distance of 200 mm from the working face of the cathode.
The advantage presented by the present invention is the availability of producing high-quality coatings on large and long articles (500 mm and more). This advantage is vital to today's industry and will make it possible to significantly broaden the range of application of vacuum coatings.
The advantage arises through the use of a vacuum-arc plasma source which incorporates a cathode with the working surface surrounded by a static magnetic stabilizing system, the lines of force of which are inclined towards the working face; primary and auxiliary anodes; a screen arranged coaxially with the cathode; as well as a high-voltage igniting electrode. The cathode is made as a rectangular plate and the magnetic stabilizing system is comprised of mutually perpendicular linear conductors which form two subsystems. The first subsystem of linear conductors is the static magnetic stabilizing system and includes conductors arranged parallel to the longer plate edges. The conductors of the second subsystem, forming the dynamic magnetic stabilizing system, are arranged at right angles to the cathode face and are divided into sections. The conductors in each section are connected to the current source through a switch, and are successively engaged in turn. The direction of current in each of the linear conductors of the second subsystem (these conductors are located closer to the cathode) is the same and coincides with the direction of current in the vacuum arc.
The linear conductors are parallel to the long sides of the cathode and interconnected by circuit closing conductors. In the case of the conductors of the first subsystem, i.e. the conductors which form the static magnetic stabilizing system, distance S between the circuit closing conductors and the side surface of the plate (cathode) and distance L between the linear conductors conform to the relationship: EQU L.ltoreq.S .ltoreq.3L
If S is less than or equal to L, the electric arc concentrates in the centre of the working face of the cathode which results in reduction of the efficiency of the cathode and in a decrease in the dimensions of the articles that can be coated.
If S&gt;3L, the efficiency of the process does not increase, and thus a further increase in the distance between the side surface of the cathode and the closing conductor is useless.
Each of the linear conductors can form a rectangular circuit. The linear conductors included in the static magnetic stabilizing system may form separate rectangular circuits located on both sides of the side surface of the cathode (see FIG. 3). The circuit closing conductors in this embodiment are located away from the cathode, to nullify their magnetic effect. In a further embodiment of the vacuum-arc plasma source the circuit closing conductors joining the linear conductors of the first group are arranged at right angles to the surface of the working face of the cathode (see FIG. 2).
All of these features are interrelated and aimed at the solution of one problem, which consists of the application of coatings to large and long articles the size of which, for example, ranges from 500 mm to 2 m and more.
The effective maintenance of cathodic spots within the working surface of the electrode can be accomplished through the use of the principles of motion of cathodic spots in the external non-uniform magnetic field. The basic principle is that the cathodic spot of a vacuum arc in a fairly strong magnetic field (.sup..about. 100 E), the force lines of which cross the surface of the cathode at acute angle .alpha., move in a direction perpendicular to the tangential component of the field in the reverse direction and, concurrently, displace towards angle .alpha. (for example, see "Cathodic Processes of Electric Arc" by Kesaev I. G., Nauka, 1968).
The cathode made in the form of a rectangular plate surrounded by the stabilizing magnetic system consisting of the dynamic and static stabilizing subsystems makes it possible to hold the cathodic spot of the vacuum arc on a flat surface dimensioned from 500 mm to 2 m and more. This, in turn, allows vacuum coatings to be produced on large and long articles of the same length.